Marketers are spending more money on Internet advertising than ever before. But so far, that isn't trickling down to the bottom lines of most of the young publicly traded online ad firms whose stock prices have tumbled in recent months.

The firms aren't household names such as Google Inc. or Facebook Inc., but smaller companies that help marketers buy ad space and deliver those ads on websites and in applications. They include automated ad-buying company Rocket Fuel Inc., online video-ad firm YuMe Inc., ad-technology...